Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 133
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 133, with English translation by Maimonides (Rambam).
להפריש חלה לכוהן, שנאמר: "ראשית, עריסותיכם--חלה, תרימו תרומה" (במדבר טו,כ).
To separate challah [and give it] to the priest, as [Numbers 15:20] states: "The first of your dough, the challah, you shall separate as an offering...."